This goes towards handling all cfd state transitions in one place, so it becomes easier to reason about it.
Currently, state transitions are scattered over the code of the cfd actor.
This change bundles the actual transition in the cfd itself.
Inserting the new state into the db is still in the responsibility of the caller.
note: the `monitor` import in `taker.rs` should technically not be part of this commit, but was needed because I decided that the `CfdMonitoringEvent` should live in the `monitor` actor. But since that is not part of the `Cfd` in `model` the taker needs to know about the `monitor`.
Lucas Soriano del Pino
Daniel Karzel